It is well known that the moisture content in wood-bodied instruments has a significant impact on an instrument's structural integrity and tonal quality. Excessive drying may cause cracks along ring-lines of the wood or at the interface of two wood sections, usually glued together. Arid conditions may also result in deformations to the soundboard, affecting the position of the “bridge” and resulting in misalignment of the strings with respect to the neck. Deformations, also referred to as bowing, bending, or warping, may also contribute to a separation between the soundboard and the bridge of a guitar or between the body and neck of most wood-bodied instruments. Therefore, it is essential to prevent the wood from drying out. A humidifier placed inside the instrument (or instrument case) acts to add water vapor to the air in and surrounding the hollow body of the instrument. The rate of humidification depends on relative humidity differences between humidified air and air near the instrument's wood surfaces.
Previous attempts to humidify hollow-body instruments have been proposed. Those attempts involve the use of a passive (or open-loop) technique that relies on assumed vapor pressure differentials with no feedback from active, accurate sensors.
Some of the prior inventions designed to address humidity concerns are moisture rich containers placed inside an instrument's protective carrying case. Perforations in the container allow moisture to leave the container and to diffuse throughout the case, ultimately entering into the hollow body of the instrument. However, as noted over 50 years ago by HOLLANDER (U.S. Pat. No. 3,407,700), the case is typically vapor permeable and permits water vapor to escape from the case. This limits the amount of water vapor that can potentially enter the hollow body of the instrument. Additionally, the moisture that remains in the case rarely enters the sound hole, and thus rarely enters the hollow body, which is a significant issue, since the most sensitive surfaces of a wood instrument to the humidity of the ambient environment are the unprotected surfaces inside the instrument. With the humidification device placed within the instrument's case, but not inside the instrument, the problem of too little moisture entering the hollow body of the instrument arises for two reasons: (1) the small size of the sound holes limits the amount of humidified air that can diffuse into the wood instrument and (2) the humidified air must travel a significant distance to contact the unprotected surfaces inside the instrument, which probabilistically decreases the chance that the humidified air will contact these surfaces. As yet another issue, the moisture content of these inventions is quickly exhausted, requiring users to refill the containers frequently. Additionally, passive humidification means may also be detrimental to the instrument, since the adjusted humidity could still remain inaccurate.
Despite the severe limitations presented above in humidifying a case, efforts of this type continued. BERLINER (U.S. Pat. No. 3,431,038) increased the water capacity. EGBER (U.S. Pat. No. 8,748,723) considered using a saturated aqueous solution encased in a semipermeable membrane. Some of the problems with semipermeable pouches include (but are not limited to): the restriction to use aqueous solutions that do not chemically react with the packaging, the potential for release of undesirable gases (such as hydrogen sulfide), a limited moisture transfer capacity (only a fraction of the moisture in the package can be used for humidification), and the need for a specific pouch for a specific environment or desired humidity range. Another problem is that use of these products involves weighing the package before installation and then periodically afterward to calculate the remaining moisture content—a prohibitively cumbersome task.
A significant improvement to humidifiers consists of placing the humidifying device inside the hollow body rather than in the case. HOLLANDER (U.S. Pat. No. 3,407,700) disclosed a snake-like structure that is inserted into the hollow body through the sound hole. Inside the device is a damp, spongy material. Placing the humidifier inside the wood instrument mitigates the problem presented above of humidified air entering the sound hole. Regardless, water vapor can still leave the sound hole and the case. In addition, these inventions require frequent maintenance of the moisture source and do not provide the user with critical details about the environment inside the hollow body. Variations to this idea include VON MYER (U.S. Pat. No. 3,721,152), LASKIN (U.S. Pat. No. 4,572,051), LIGHT (U.S. Pat. No. 5,289,751), and HEPPLE (U.S. Pat. Nos. 8,087,645/8,220,782).
BLACKSHEAR (U.S. Pat. No. 4,649,793) improved the prior state of the art by proposing a mechanism to seal off the hollow body in order to mitigate the effect of moisture loss to the ambient environment. His detailed investigation into moisture loss mechanisms revealed that moisture can penetrate the lacquer or epoxy finish on the outside of an instrument. Therefore, even instruments with the sound hole covered require a humidification device with a large water reservoir, since the water vapor can still escape the enclosed internal environment.
Blackshear also noted the deleterious effect of sudden changes in humidity. “Because wood is viscoelastic, i.e., it can flow as well as elastically deform, greater swings in humidity and the resulting volume changes can be tolerated without crack formation if the changes occur gradually.” However, a shock to the system from sudden changes in humidity can ultimately lead to large stresses in the wood resulting in cracks. Although Blackshear mentioned moisture shock in his discussion, his device does not actively control the actual rate of humidification.
The remaining effort in Blackshear's work was in building an improved humidification system that operates under specific but limited ambient conditions. The specificity of this device limits its operation to a narrow range of ambient conditions. In addition, the sponge of this device still needs to be re-wet weekly.
None of the aforementioned inventions provide an indicator of the actual humidity level inside the instrument body. The commercial product Planet Waves Acoustic Guitar Humidifier by D'Addario has a temperature and humidity sensor and display. In addition, it suspends into the hollow body via the strings, and it is filled with a damp, spongy material. However, this product does not cover the sound hole. Furthermore, the output of the Planet Waves humidity sensor does not affect the output of the moist sponge. Consequently, this system is passive and the humidity sensor is useless unless the user regularly monitors it.
In fact, all of the previously mentioned inventions are passive (open-loop) systems. Terms used in prior patents such as “auto regulation” or “self-regulating” are misnomers that imply that these devices incorporate active closed-loop humidity regulation. Instead, these devices stop producing an output when vapor pressure reaches equilibrium, not when a user-defined set point is reached, which may be above or below the equilibrium value. Furthermore, the majority of these devices do not measure the humidity of the environment.
The inventor, Shengxie Nangong (CN 201804543 U), combined some of the features of the prior art to produce a sound-hole cover with a snake-like sponge that penetrates into hollow body. There is also a hydrometer that displays the current humidity level. However, like the other prior art, this system is passive, is incapable of achieving a desired set point, and needs to be maintained frequently.
In summary of the prior art: the performance of the prior art is heavily bounded by initial ambient conditions (i.e. if the initial ambient humidity is too low, an equilibrium may never be reached), the permeability of the instrument, the permeability of the case (if used), and the diligence of the user to frequently inspect the humidifying device; many of the semipermeable pouches contain toxins or corrosive elements; the moisture transfer capacity of the prior art is low; most devices do not display the actual humidity level in the hollow body or the case; the prior art does not control the humidification rate to address moisture shock, and the prior art does not actively control the output of the moisture-generating device.